1. Field
Aspects of the present invention relate generally to the field of video coding and more specifically to image detail and edge enhancement.
2. Description of the Related Art
An edge enhancer comprising both fine detail enhancement (detail EE) and luminance transition improvement (LTI) may be used to improve the transitions in a video signal. Applied in parallel, detail EE on the small transitions and LTI on the large transitions, the output of an edge enhancer is the sum of the original signal (Yin), the detail enhancement (ΔYdet) and the transition improvement (ΔLTI). Detail EE uses the principles of un-sharp masking and causes overshoot or ringing on edges that can enhance small details.
Up-scaling is typically necessary when an input video signal is standard definition (SD), often 480i, and the desired output is high definition (HD), usually 1080i or 1080p. Up-scaling has the effect of enhancing artifacts or imperfections that may exist or be introduced in the video signal. Therefore, when up-scaling a signal, it may be important to verify that the actual details in the video image are enhanced rather than enhancing and propagating imperfections.
When a signal is up-scaled, detail EE performance may be compromised because the overshoot and ringing caused by the enhancement can result in added mosquito noise and augment stray artifacts in the image. Additionally, up-scaling often results in a softening of the image and of the finer details because the higher frequencies in the un-scaled video translate to lower frequencies in the up-scaled video. To implement an effective detail EE after scaling (post-scaler), a new filter design would be required to enhance the frequencies where the real details exist after scaling and would result in a much larger filter that would need to change based on the up-scaling ratio. Therefore detail EE is more effective when implemented before up-scaling.
LTI is a non-linear algorithm for improving transition sharpness on large amplitude edges without causing overshooting or ringing. However, LTI does not noticeably improve small transitions. LTI may artificially introduce frequencies that can create artifacts and introduce aliasing or pixellations known as “jaggies” so LTI is most effective when applied post scaler because the up-scaled signal's frequency spectrum contains space for the added frequencies. Furthermore, the added artifacts and jaggies are more obvious after the enhanced video is up-scaled. Therefore, when an edge enhancer comprising both detail EE and LTI is applied pre-scaler, the LTI enhancement does not work as expected, and when applied post-scaler, the detail EE does not work as expected.